A need exists for a fuel-flexible burner for refineries, chemical plants, and other facilities which will enable the operation of fired heaters using fuels ranging from conventional gases to bio-gases and synthetic gases. The burner will preferably be effective for safely and efficiently burning a broad range of gaseous fuels in a cost-effective manner while also minimizing emissions of pollutants. In addition, the burner will preferably provide a flame stabilization mechanism which will allow the burner and the fired heating system to quickly and safely adapt to sudden and wide swings in the heating value of the fuel delivered to the burner.
In a petroleum refinery, the composition of the refinery fuel gas generated by the refinery operations will vary considerably, and can change suddenly, depending upon the refinery configuration and upon the operating status and characteristics of the numerous processing units within the refinery. For example, Flexicoker off-gas is a low-BTU gas which is produced and used in many refineries and which can significantly reduce the heating value of the fuel delivered to the burner if used separately or in combination with other gases.
Heretofore, when the heating value or supply of the refinery fuel gas has been low, natural gas has typically been blended with the refinery-generated gases to supply the balance of the plant's energy requirements. Alternatively, natural gas can serve as a dedicated fuel for a unit or an entire plant.
Additional gaseous fuels of interest for use in fired-heaters include biogas from organic matter digesters, including animal and agricultural wastes, waste water plants, and landfills; as well as syngases from the gasification of biomass, municipal solid wastes, construction wastes, or refinery residuals such as tar, pitch and petroleum coke. Unfortunately, however, these gases typically have very low heating values and can vary significantly in composition.
The degree of interchangeability of gaseous fuels for use in combustion applications can be evaluated by determining and comparing the Wobbe numbers of the fuels in question. The Wobbe number of a gaseous fuel is determined by dividing the higher heating value of the fuel by the square root of its specific gravity. For incompressible flow through a fixed fuel orifice with constant fuel supply pressure, the energy flow rate (i.e., firing rate) of a gas fuel will be proportional to its Wobbe number.
Typically, the Wobbe number values for the different types of gas fuels mentioned above are as follows: from 120 to 150 for syngas; from 500 to 600 for biogas; from 1300 to 1400 for natural gas; and from 1100 to 1500 for refinery fuel gas. Consequently, in order to be able to use all of these various types of fuels interchangeably in one combustion system, the combustion system would be required to accommodate over an order of magnitude of variation in the Wobbe number value of the fuel delivered to the burner.
Heretofore, the burners available in the art have not been able to adequately and effectively respond and adapt to heating value and Wobbe number value changes approaching this magnitude. In fact, most commercial burners currently in service are not capable of handling low heating value fuels such as biogas and syngas at all. The stability mechanisms of the burners currently available in the market are typically designed for fuels that burn much more readily. Moreover, rapidly changing from one fuel to another stresses the stability of the burner even further.
Consequently, biogases, syngases, and other such low heating value gases are commonly viewed as being essentially unusable and as being so difficult to burn in a stabilized manner that they are simply flared off, thus wasting the energy content of these gases and leading to an increase in greenhouse gas emissions.